How to adjust a specular map's strength?
You cannot directly set the value - it is generated by the connected *.PNG file.

However, you can make the effect stronger or weaker.
Add math node in front of it and set it to multiply.
This works for most situations.

Just like this:


Important Note:
Avoid connecting textures into specular input, if you’re going for physically accurate result.
Use metalness texture instead and connect it to metalness input. And keep the default specular value of 0,5.
(CG Cookie explained it best: https://youtu.be/mrNMpqdNchY?t=244)

If something ain’t clear, let me know. Specular input is kinda weird, I don’t want to overwhelm you.

Hope this helps.